Secondary batteries have come into wide use not only for power supplies of portable devices such as cellphones, digital cameras, and laptop computers, but also for domestic power supplies or vehicle power supplies. Particularly, a lithium-ion secondary battery, which has high energy density and light weight, has become an energy storage device necessary for life.
Secondary batteries are classified broadly into a rolled type and a stacked type. A battery electrode assembly in a rolled type secondary battery has a structure in which a long positive electrode sheet and a long negative electrode sheet that are separated from each other by a separator, are stacked and rolled a plurality of times. A battery electrode assembly in a stacked type secondary battery has a structure in which positive electrode sheets and negative electrode sheets are stacked alternately and repeatedly with separators interposed therebetween. Each of the positive electrode sheets and the negative electrode sheets includes a current collector having a coated portion to which active material (which may be a composite material including a binding agent, a conductive material or the like) is applied and an uncoated portion to which no active material is applied in order to connect with an electrode terminal.
In either a rolled type secondary battery or a stacked type secondary battery, a battery electrode assembly is sealed in an exterior container (an exterior case) such that one end of a positive electrode terminal is electrically connected to an uncoated portion of a positive electrode sheet and the other end extends to the outside of the exterior container, one end of a negative electrode terminal is electrically connected to an uncoated portion of a negative electrode sheet and the other end extends to the outside of the exterior container. Electrolyte is also sealed in the exterior container. The trend in secondary batteries is towards capacity increases. Increased battery capacities involve an increase in heat generation if cases when a short circuit occurs and this may significantly raise the danger threshold. Therefore, the adoption of appropriate safety measures has become increasingly important.
One example of a safety measure is a technique of forming an insulator at a boundary between a coated portion and an uncoated portion in order to prevent a short circuit between a positive electrode and a negative electrode (Patent Document 1).